Engine instrumentation is most useful when readings are readily translatable into information that helps the operator operate efficiently and safely. Where the importance of continual and reliable engine performance is high, such as in airplanes, boats or the like, accurate and useful information becomes critically important to the safety of operators and passengers. Typically, as the need for better information increases, instrumentation becomes more complex and less familiar to inexperienced operators. In many situations, instrumentation may offer information in a form that, while clearly readable, has little or no meaning to the inexperienced operator.
A simple example of such instrumentation is the tachometer, reporting engine performance in terms of output shaft revolutions per minute (RPMs), which is familiar in boats, airplanes, and high-performance automobiles. While the neophyte operator of the more complex engine systems may know that it is dangerous to run an engine at an excessive RPM level, and that the tachometer output is somehow related to speed, he may not know the nature of the relationship between engine RPMs and the power output of the engine. Since engines differ somewhat in their power operating curves, even an experienced operator may not understand the exact relationship, for a particular engine type, between efficient levels of performance and various RPM settings. For both operators, it would be useful if the instrument and its means of display informed the operator not only of the actual operating value measured, but also how the particular reading relates to critical matters of operating safety and efficiency.